The present invention relates to an optical single crystal film, a process for producing the same and an optical element comprising the same.
Description of Related Art
It is well-known to develop blue laser light sources for optical recording and reading-out with a high density, for example, second-harmonic generation (SHG) devices using a lithium niobate or lithium tantalate single crystal.
For example, according to "Journal of Applied Physics" Vol. 70, No.5, p.2536-2541, (September 1991), there is disclosed the formation of a lithium niobate film on a MgO doped lithium niobate substrate by means of a liquid phase epitaxial process using Li.sub.2 O--V.sub.2 O.sub.5 flux. The optical propagation loss of this film, however, is extremely large, for example, as large as 25 dB/cm at a green light of 514.5 nm wavelength. This defect is attributable to optical absorption caused by a V ion introduced into the film from the flux. It further describes that in order to decrease the optical absorption caused by the V ion, it is necessary to change the valence number of V ion from +3 to +5. To accomplish this, an ozone annealing treatment is employed. Especially, it reports that the optical propagation loss at the green light of 514.5 nm wavelength can be improved to 1.6 dB/cm, by annealing a once produced lithium niobate film at 600.degree. C. for an hour in an ozonic atmosphere. However, it also discloses that even with the ozone annealing treatment, the optical propagation loss is at least 10 dB/cm, for example, in the case of a blue beam of 450 nm wavelength.
According to, the "Journal of Crystal Growth" 132 (1993), p.48-60, a lithium niobate film is formed by means of a liquid phase epitaxial process using Li.sub.2 O--V.sub.2 O.sub.5 flux and subjected to an ozone annealing treatment. Although this process highly improves an optical absorption in a green light range, the effect is not sufficient in a blue light range with shorter wavelengths. Accordingly, in order to use the film in a blue light range, it is effective to form a lithium niobate film by using, for example, a Li.sub.2 O--B.sub.2 O.sub.3 flux that is free from transition metal elements such as V and the like causing the absorption.
According to "Journal of Applied Physics" Vol.67, No.2, 15, (January 1990) p.949-954, a Bi-substituted magnetic garnet is formed by means of a liquid phase epitaxial process using a Bi.sub.2 O.sub.3 flux. The Bi-substituted magnetic garnet is then subjected to an ozone annealing treatment, for example, at 700.degree. C. for three hours and thereafter to an oxygen annealing treatment at 500.degree. C. to decrease the optical propagation loss at a 1.3 .mu.m wavelength to 1 dB/cm or less.
As mentioned above, in either "Journal of Applied Physics" Vol.70, No.5 (September 1991) or "Journal of Crystal Growth" 132 (1993) p.48-60, they failed to sufficiently decrease the optical absorption in the blue light range.
Additionally, the description of "Journal of Applied Physics" Vol.67, No.2, p.15 relates to the decrease of the optical absorption in an infrared range of, for example, 1.3-1.5 .mu.m wavelengths and not to the decrease of the optical absorption in the blue light range of 400-500 nm wavelengths.